One of the reasons for the mechanical failure occurring with optical communication fibers over extended periods of use is caused by the fatigue of the fibers under prolonged subjection to stress along the axis of the fiber. The failure mechanism over extended periods of stress is in part due to the presence of water molecules and hydroxyle groups on the glass surface of the fibers.
With glass-on-glass optical fibers wherein the core comprises a glass material and the cladding comprises a glass material, the presence of water molecules on the outer glass surface and residue stresses cause the glass network on the outer surface to become substantially weakened over a period of time so that the fiber ruptures under stress forces that are incapable of causing the fibers to fracture in the absence of water or water vapor.
In the fiber drawing operation the glass-on-glass optical fiber is coated with a silicone resin immediately after drawing and a thermoplastic resin is subsequently coated over the silicone material. Although the silicone material is effective to prevent dust particles from contacting the outer glass surface, the silicone material is relatively porous to water. Over a period of exposure in air, at ordinary concentrations of water vapor, water is able to permeate through the silicone layer and interacts with the outer glass surface.
The same mechanism of fatigue failure occurs with plastic clad silica optical fibers wherein the core material comprises silica and the silicone material comprises the cladding layer. Although the extruded thermoplastic jacket covering the silicone material retards the permeation of water through the silicone layer to some degree, water can permeate through the jacket to the silicone material and from there through to the glass surface.
One of the purposes of this invention therefore is to provide an interfacial water impervious layer to the glass optical fiber surface.